This invention relates generally to the heating of water and more particularly to a method and apparatus for using waste heat to enhance the efficiency of fuel fired hot water heaters.
Domestic hot water heaters which use natural gas, fuel oil, or propane as a fuel typically have a pilot burner which burns continuously so that it can ignite the main burner when heating of the water is required. The heat from the pilot light is in large part lost through the furnace flue and is thus essentially wasted energy which does not contribute significantly to heating of the water. The temperature of the flue in the water heater is normally in the range of about 100.degree. F. to 150.degree. F. when only the pilot light is burning. When the main burner is ignited, the flue temperature is raised to about 450.degree. F.-400.degree. F. Although various measures have been taken to more efficiently use the heat of the flue gases, such as providing spiral baffles in the flue, a large amount of heat is still lost through the flue, and the efficiency of the water heater suffers accordingly.
It is the principal goal of the present invention to increase the efficiency of fuel fired hot water heaters by making use of what is normally waste heat.
More specifically, it is an object of the invention to provide a method and apparatus for using heat from the pilot burner to preheat water which is supplied to the water heater tank, thereby decreasing the heating requirements of the main burner to reduce the fuel use.
Another important object of the invention is to provide a method and apparatus for extracting heat from the flue gases and using the heat for preheating of the water before it is delivered to the main water heater tank.
A further object of the invention is to provide a preheating system which can be supplied either as original equipment along with a hot water heater or as add-on equipment that can be quickly and easily connected with an existing hot water heater.
An additional object of the invention is to provide a preheating system in which flushing of the conduits, either in the direction of normal flow or in the reverse direction, occurs automatically when hot water is removed from the main tank.
In accordance with the invention, an auxilliary water storage tank is provided side by side with a conventional fuel fired water heater. The incoming cold water that is to be heated is initially delivered to the storage tank. In one embodiment of the invention, a small pump operates continuously to pump the water from the storage tank through a conduit loop which extends through the flue of the main tank and through a special heat exchanger located adjacent to the pilot burner. In another embodiment, circulation occurs naturally due to thermal effects. In either arrangement, the water which is heated in the conduit loop is returned to the storage tank and stored until required by the main tank. Then, a transfer conduit transfers the preheated water from the storage tank to the main tank, and the preheating of the water reduces the heating requirements of the main burner.
The sections of the conduit loop that extend through the flue and heat exchanger are preferably corrugated pipe in order to enhance the heat transfer to the water that is circulated by the pump. The sections of the conduit in the flue are arranged in a spiral configuration to increase the residence time of the hot combustion gases in the flue and also to provide a large surface area for transfer of heat to the conduit. The heat exchanger can take the form of an open bottom hood located immediately above the pilot burner to concentrate the heat that is generated by the pilot flame. Preferably, the conduit loop is arranged such that it is automatically flushed to prevent the accumulation of debris.